Laser printing system

ABSTRACT

The invention describes a laser printing system (100) for illuminating an object moving relative to a laser module of the laser printing system (100) in a working plane (180), the laser module comprising at least two laser arrays of semiconductor lasers and at least one optical element, wherein the optical element is adapted to image laser light emitted by the laser arrays, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in the working plane of the laser printing system, and wherein the laser printing system is a 3D printing system for additive manufacturing and wherein two, three, four or a multitude of laser modules (201, 202) are provided, which are arranged in columns (c1, c2) perpendicular to a direction of movement (250) of the object in the working plane (180), and wherein the columns are staggered with respect to each other such that a first laser module (201) of a first column of laser modules (c1) is adapted to illuminate a first area (y1) of the object and a second laser module (202) of a second column (c2) of laser modules is adapted to illuminate a second area (y2) of the object, wherein the first area (y1) is adjacent to the second area (y2) such that continuous illumination of the object is enabled.

FIELD OF THE INVENTION

The invention relates to a laser printing system and a method of laserprinting, namely in the field of 3D printing by means of lasers foradditive manufacturing, for example, used for rapid prototyping. Theinvention does not refer to 2D printing such as printing of documents.

BACKGROUND OF THE INVENTION

Selective-laser melting machines consist of a single high-power laserand a scanner to scan the laser over the area to be illuminated. In suchmachines the laser and the scanner are arranged outside of a processchamber and the laserlight can pass through an entrance window into theprocess chamber that contains a building area. To increase theprocessing speed, it is desirable to have a printing head with severalindependent channels i.e. an addressable array of lasers covering asignificant part of the area. Preferably, the printing head covers thefull width of the area to be printed with one addressable laser sourceper pixel, so that the print head needs to be moved only in onedirection. Reliability and service costs of such addressable arrays maybe an issue.

US 2005/0151828 A1 discloses a device for xerographic laser printing.The xerographic printing system has a laser printbar imager assemblyincluding a plurality of micro-optic light emitting arrays. Themicro-optic light emitting array includes a plurality of vertical cavitysurface emitting lasers, where each vertical cavity surface emittinglaser is focused with a micro-optic element.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improvedlaser printing system and a corresponding method of laser printing.

According to a first aspect a laser printing system for illuminating anobject moving relative to a laser module of the laser printing system ina working plane is provided. The laser module comprises at least twolaser arrays of semiconductor lasers and at least one optical element.The optical element is adapted to image laser light emitted by the laserarrays, such that laser light of semiconductor lasers of one laser arrayis imaged to one pixel in a working plane of the laser printing system.

Preferably, an area element of the pixel is illuminated by means of atleast two semiconductor lasers.

Known laser printing systems either use single high power lasers orarrays of lasers. In case of high power lasers, for example, a singleedge emitting semiconductor laser may be used whereas in case of laserarrays Vertical Cavity Surface Emitting Lasers (VCSELs) are preferablyused. VCSEL arrays can be easily manufactured in wafer based processesbut usually emit less power than edge emitting semiconductor lasers. Theoptical systems of these known laser printing systems project or focusthe light emitting layer of each semiconductor laser to the workingplane.

In contrast to this approach a preferred embodiment of the presentinvention proposes to image at least two laser arrays to two pixels inthe working plane by means of an optical element. The image of the laserarrays does not comprise sharp images of the light emitting layers ofthe semiconductor lasers. Light emitted by means of at least two lasersof one of the laser arrays illuminate each area element of the pixelsuch that there is no area element which is only illuminated by means ofone single semiconductor laser. Preferably, three, four or a multitudeof semiconductor lasers of one laser array illuminate one area elementof a pixel at the same time. It may even be that two laser arrays areimaged to the same pixel at the same time. More intensity can thus beprovided to the working plane by using a multitude of semiconductorlasers per area element of a pixel. A diffuse image of a multitude ofsemiconductor lasers of the arrays form the pixels in the working plane.The laser printing system may be more reliable due to the relatively lowcontribution of each single semiconductor laser to the illumination orenergy input to an object in the working plane by means of opticalenergy. A malfunction of a single semiconductor laser of a laser arraydoes consequently not cause a malfunction of the laser printing system.

The laser module may move relative to the laser printing system(scanning) and/or the object may move relative to the laser printingsystem. The object may be a powder layer which can be sintered by meansof the laser printing system. It may be preferred that only the objectis moved. The laser printing system may be enabled to illuminate thefull width of the object moving perpendicular to the width of the objectby means of one, two, three, four or more laser modules. Thesemiconductor lasers may be edge emitting semiconductor lasers but VCSELarrays may be preferred due to lower costs.

The optical element may be arranged in a way that an object plane of theoptical element with respect to the working plane does not coincide witha plane of the semiconductor lasers such that the cones of laser lightemitted by adjacent semiconductor lasers overlap in the object plane.The plane of the semiconductor lasers of the laser arrays is defined bymeans of the light emitting layers of the semiconductor lasers. Thelight emitting layers comprise the optical cavity of the semiconductorlasers comprising the active layer and the corresponding resonatormirrors. The optical element may be a single imaging lens or a morecomplex imaging optic defining an object plane relative to the workingplane. The arrangement of the object plane with respect to the lightemitting layers of the semiconductor lasers of the laser arrays maycause a diffuse overlapping image of the light emitting layers in theworking plane. The energy distribution in the working plane may thus bemore homogeneous in comparison to a projection of each light emittinglayer of the semiconductor layers to the working plane. Furthermore, theoptical element may be as simple as one projection lens per laser modulebut more complex combinations of lenses may be used in order to increasethe distance between the working plane and the laser modules. Nomicro-lens arrays may be needed in order to provide a sharp projectionof each light emitting layer.

The laser module or laser modules of the laser printing systempreferably comprises three, four or a multitude of laser arrays. Asingle laser array may be imaged to one pixel in the working plane. Thepixels may be adjacent to each other such that a part of the emittedoptical power of one laser array overlaps with the optical power emittedby another laser array. It may even be that two three or more laserarrays may be mapped to the same pixel in the working plane. The opticalelement may comprise an array of micro-optical elements which may image,for example, laser light of two, for example, adjacent arrays of thelaser module to one pixel in the working plane. Two or more arrays mayin this case be imaged to one pixel. Alternatively or in addition it maybe that laser light emitted by different laser arrays may illuminate thesame part of a surface of an object at different times. The latter meansthat light of a first array may illuminate a defined surface of theobject at a time t₁ and light of a second array may illuminate thedefined surface of the object at a time t₂ later than t₁ wherein theobject was moved relative to the laser module(s). Furthermore, theprinting system may comprise laser modules with different workingplanes. The latter may be done by placing laser modules at differentheights relative to a reference surface and/or by providing differentoptical elements. Different working planes may be advantageous for threedimensional printing. Alternatively or in addition it may be that thelaser module(s) can be moved with respect to the reference surface beingparallel to the working planes being always at a defined distance withrespect to the laser modules.

The laser arrays of the laser module or laser modules may be arranged incolumns perpendicular to a direction of movement of the object in theworking plane. The columns may be staggered or cascaded with respect toeach other such that a first laser array of a first column of laserarrays is adapted to illuminate a first area of the object and a secondlaser array of a second column of laser arrays is adapted to illuminatea second area of the object, wherein the first area is adjacent to thesecond area such that continuous illumination of the object is enabled.The images of the laser arrays may partly overlap as discussed above.

The laser arrays may be rectangular with the long side of the rectanglebeing arranged parallel to a direction of movement of the object in theworking plane. This arrangement allows higher total powers per pixel byproviding more semiconductor lasers per pixel, without reducing theresolution in the lateral direction perpendicular to the direction ofmovement of the object.

According to the invention, the laser printing system comprises two,three, four or a multitude of laser modules. Using a multitude of lasermodules may enable a larger printing area. Furthermore, complex opticalelements may be avoided by using, for example, one imaging lens perlaser module.

Further, the laser modules are arranged in columns perpendicular to adirection of movement of the object in the working plane. The columnsare staggered or cascaded with respect to each other such that a firstlaser module of a first column of laser modules is adapted to illuminatea first area of the object and a second laser module of a second columnof laser modules is adapted to illuminate a second area of the object,wherein the first area is adjacent to the second area such thatcontinuous illumination of the object is enabled.

The number of columns of laser modules may be arranged in a way that thedistance between laser modules in one column of laser modules isminimized. The module diameter and the width of the image of the arraysmay determine the number of columns needed in order to enable an areacovering illumination of the object by means of the laser modules. Thebigger the diameter of the module in relation to the width of the imageof the array arrangement the more columns may be needed.

The laser arrays of each laser module may be arranged in an elongatedarrangement with the long side of the elongated arrangement beingarranged perpendicular to the direction of movement of the object in theworking plane. Each laser module may comprise, for example two, three ormore columns of laser arrays perpendicular to the direction of movementof the object in the working plane. The number of arrays per column maysurpass the number of columns. This arrangement may enable a homogeneousillumination of the object by means of a relatively simple drivingscheme of the single arrays especially if more than one laser module iscomprised by the laser printing system. Each area element of the objectmay in this case only be illuminated by one dedicated laser arraywherein adjacent laser arrays illuminate adjacent pixels. The velocityof movement of the object in the working plane may be adapted in orderto define the total energy per area element of the object.

The laser printing system may comprise two, three, four or a multitudeof laser modules wherein the laser arrays of each laser module arearranged in an elongated arrangement in order to enable a broadworkspace (printing width perpendicular to the direction of movement ofthe object) of the laser printing system.

The laser arrays of each laser module may alternatively be arranged inan elongated arrangement with the long side of the elongated arrangementbeing arranged slanted or rotated with respect to the direction beingperpendicular to the direction of movement of the object in the workingplane. A defined slanting angle or rotation of the elongated arrangementof laser modules around their centers may enable integrated intensityprofiles with smooth slopes, which may also overlap with the adjacentpixels, to improve the homogeneity of the total intensity distribution,especially if the pixels are slightly misaligned with respect to eachother. The latter reduces the alignment effort of the laser arrays andthus the manufacturing costs of the laser modules and the laser printingsystem. The misalignment may in extreme cases be compensated by anadditional calibration run of the laser printing system in which thevelocity of movement of the object in relation to the energy input pertime and area element of a calibration object is determined.

Alternatively, two, three or more laser arrays of the same laser moduleor different laser modules may be arranged to illuminate the same areaelement of the object. The laser arrays may be arranged to subsequentlyilluminate the area element. The energy input per time to an areaelement of the object in the working plane may be increased. This mayenable higher velocities of the object and thus higher throughput of thelaser printing system. Furthermore, the tolerance regarding misalignmentof laser arrays and malfunctions of single semiconductor lasers may beimproved. The driving schemes of different arrays may be adapted basedon calibration runs with calibration objects as described above.

The optical element of the laser modules may be arranged to demagnifythe image of the laser arrays in the working plane. The demagnificationmay enable a smaller pixel size and higher energy densities. Each laserarray may further comprise a micro-lens array being part of the opticalelement, the micro-lens array may be arranged to lower the divergence ofthe laser light emitted by the semiconductor lasers. The reduction ofthe divergence may be used to find a compromise between an overlap ofthe laser light emitted by the semiconductor lasers in the object planeand the size of the single pixel. Furthermore, the distance between thelaser array and the working plane may be adapted by means of themicro-lens array and/or the optical element (imaging optic) may besimplified.

The density of laser arrays may change in dependence of an area of theobject to be illuminated by means of the laser printing system. Thelatter may enable higher power densities at defined parts of the object.Alternatively or in addition the density of the semiconductor laserswithin the arrays may be adapted such that, for example, less or moreintensity may be provided at the rim of the pixels. Furthermore, theshape of the arrays may be tailored in order to improve the homogeneityand/or to create a defined intensity distribution in the working plane.The arrays may, for example, have a diamond, triangle, round,elliptical, trapezoid or parallelogram shape.

The laser printing system comprises at least a first and a second lasermodule arranged next to each other. Each laser module may comprise atleast two laser arrays, wherein at least one of the two laser arrays ofthe first or the second laser module is arranged as an overlap laserlight source such that in operation the same area element in the workingplane can be illuminated by the overlap laser light source and a laserarray of the laser module arranged next to the laser module comprisingthe overlap laser light source.

The overlap laser light source is arranged to compensate potentialmisalignments of the laser modules which may result in unintendedillumination gaps on the object in the working plane. Therefore theoverlap may be partial.

The laser arrays may illuminate each one pixel in the working plane. Thelaser array which is arranged as overlap laser light source may bearranged to illuminate the same pixel or a part of the same pixel as alaser array of the neighboring laser module. This means that both laserarrays can illuminate the same area element in the working plane at thesame moment in time. Alternatively, the overlap laser light source maybe arranged to illuminate the same area element as a laser array of theneighboring laser module but later or earlier in time. The light of theoverlap laser light source may, for example, illuminate one area elementof an object in the working plane at a time t1 and the laser array ofthe neighboring laser module may illuminate the same area element at atime t2 later than t1 because of the movement of the object relative tothe laser modules. The relative movement may be caused by a movement ofthe object, a movement of the laser modules or movement of the objectand the laser modules. The total intensity which is provided to adefined area element of the object has to be adapted such thatessentially the same energy is provided per area element as in the caseof perfectly aligned laser modules which wouldn't need an overlap laserlight source. The energy which is provided per area element has to beadapted such that defects in the object are avoided. Only the overlaplaser light source or the laser array of the neighboring laser modulemay be used if there is a perfect match between the illuminated areas.Alternatively, both may be used with adapted intensity (e.g. 50%intensity) wherein the adapted intensity may be adapted to the relativevelocity of the object with respect to the laser module. The adaption ofthe provided laser light may be important if there is no perfect matchbetween the illuminated area elements (e.g. only half overlap due tomisalignment) in order to avoid that too much or too little energy isprovided.

The technical measures as described in dependent claims 2 to 13 and thecorresponding description may be combined with the overlap laser lightsource as described above.

A total energy which is provided to at least one defined area element inthe working plane may be such that essentially the same energy isprovided per area element as in the case of aligned laser moduleswithout an overlap laser light source.

In addition, a total energy which is provided to at least one definedarea element in the working plane may be such that essentially the sameenergy is provided per area element as in the case without a time offsett2−t1 between the illumination of the at least one defined area elementby the laser array and the corresponding overlap laser light source.

The adapted intensity of a laser array and/or a corresponding overlaplaser light source may be such that an energy loss in the time betweenthe illumination by the laser array and the illumination by thecorresponding overlap light source of a defined area element in theworking plane that is illuminated by the laser array at a time t1 and bythe overlap laser light source at a time t2 or vice versa iscompensated.

The adapted intensity of a laser array and/or a corresponding overlaplaser light source may be selected depending on a building material usedfor 3D printing.

In a laser system which is not claimed comprising an overlap laser lightsource, laser light sources as single lasers may be used instead oflaser arrays as described above. The technical measures as described independent claims 2 to 15 and the corresponding description may becombined with the overlap laser light source in the laser systemcomprising single lasers (instead of laser arrays) if applicable.

One pixel may be illuminated by a multitude of semiconductor lasers of alaser array at the same time and the total number of semiconductorlasers may be such that failure of less than a predetermined number ofthe semiconductor lasers reduces the output power of the laser arrayonly within a predetermined tolerance value. This avoids that therequirements with respect to the working life of the semiconductorlasers is unnecessarily increased.

A laser module may be configured to illuminate at least 2, morepreferably 4, 8, 16, 32, 64 or more pixels using a single opticalelement associated with the laser module.

The optical element associated with a laser module may have an outercontour obtained from a circular or rotationally symmetrical contourwhich is truncated on two opposing sides and wherein the opposing sidesare aligned with respect to each other along an axis which is preferablyoriented in a direction perpendicular to the direction of movement. Bymeans of this a compact design of an illumination unit comprising aplurality of modules which are staggered in the direction of movementcan be achieved.

A control device may be provided which controls the semiconductor lasersindividually or the laser array in such a manner that a semiconductorlaser or a laser array which is not used for illuminating is used forproviding heat to the working plane.

The semiconductor laser or a laser array which is not used forilluminating may be operated with a lower power than a semiconductorlaser or a laser array which is used for illuminating.

At least two semiconductor lasers of one laser array or at least twosub-groups of semiconductor lasers of one laser array may beindividually addressable such that an output power of the laser array iscontrollable by switching off one or more semiconductor lasers or one ormore sub-groups of semiconductor lasers. This allows to perform variousfunctions with the respective laser array, such as to use a laser arrayfor heating without melting or sintering of the building material or toprovide a required intensity in the case of overlap laser light sources.

A plurality of semiconductor lasers forming an array may be arrangedsuch that an outer contour of the array has a substantially polygonal,preferably a substantially hexagonal shape. With such a design theintensity distribution of the array is substantially free from sharpedges.

According to a still further aspect, a 3D-printing system includes aprocess chamber comprising a support for layers of a material, whereinthe laser modules are arranged in the process chamber and whereinpreferably a protective device is arranged at a side of the lasermodules which faces the support.

The protective device may be formed of at least one plate that istransparent for the laser light, preferably at least one glass plate.The protective device protects the optical elements and light sourcesand keeps the laser modules free from vapors and condensates.

A temperature control device may be provided that controls thetemperature of at least the surface of the protective device orientedtowards the support.

The temperature control device may be configured to heat the protectivedevice such that thermal radiation from material in the working plane tothe protective device is substantially prevented and the heat transportinto the illumination unit ist prevented as good as possible.

The 3D-laser printing system may be configured to solidify a materiallayer by layer at locations corresponding to the cross section of anarticle to be formed in each layer using the laser modules.

The material may be a powder.

The laser modules form an illumination unit and the illumination unitmay be configured to move across the working plane.

One laser array can be comprised of a single semiconductor laser but mayinclude at least two seminconductor lasers.

The semiconductor lasers may comprise VCSELs (Vertical Cavity SurfaceEmitting Lasers) and/or VECSELs (Vertical External Cavity SurfaceEmitting Lasers).

According to a further aspect of the present invention a method of laserprinting is provided. The method comprises the steps of:

-   -   moving an object in a working plane relative to a laser module;    -   emitting laser light by means of a laser module comprising at        least two laser arrays of semiconductor lasers and at least one        optical element; and        imaging the laser light emitted by the laser arrays by means of        the optical element, such that laser light of semiconductor        lasers of one laser array is imaged to one pixel in a working        plane. Preferably, an area element of the pixel is illuminated        by means of at least two semiconductor lasers.

The method may enable a more homogeneous intensity distribution in theworking plane.

The method may comprise a further step of moving the laser module(s)perpendicular to a reference plane being parallel to the working plane.The movement perpendicular to the reference plane enables differentworking planes which are parallel to each other.

According to a further aspect, the laser printing system used in themethod is a 3D printing system for additive manufacturing and two,three, four or a multitude of laser modules are used which are arrangedin columns perpendicular to a direction of movement of the object in theworking plane, and wherein the columns are staggered with respect toeach other such that a first laser module of a first column of lasermodules is adapted to illuminate a first area of the object and a secondlaser module of a second column of laser modules is adapted toilluminate a second area of the object, wherein the first area isadjacent to the second area such that continuous illumination of theobject is enabled.

A powder material which transforms under the influence of radiationemitted by the semiconductor lasers, e.g. melts or sinters, may be usedin the method.

The method may further include a step of moving an illumination unitacross the working area.

It shall be understood that the laser printing system of claim 1 and themethod of claim 33 have similar and/or identical embodiments, inparticular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention canalso be any combination of the dependent claims with the respectiveindependent claim. In particular, the method can be performed with thelaser printing system according to any of claims 1 to 32.

Further advantageous embodiments are defined below.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

The invention will now be described, by way of example, based onembodiments with reference to the accompanying drawings.

In the drawings:

FIG. 1 shows a principal sketch of a first laser printing system.

FIG. 2 shows a section of the first laser printing system.

FIG. 3 shows a principal sketch of a section of a second laser printingsystem.

FIG. 4 shows a principal sketch of an arrangement of laser arrays in alaser module of the laser printing system.

FIG. 5 shows a principal sketch of a first arrangement of laser modulesof the laser printing system.

FIG. 6 shows a principal sketch of a second arrangement of laser modulesof the laser printing system.

FIG. 7 shows an integrated intensity profile with every second pixel offin the arrangement of laser modules shown in FIG. 6.

FIG. 8 shows an integrated intensity profile with an arbitrary patternof on/off switched pixels in the arrangement of laser modules shown inFIG. 6.

FIG. 9 shows a principal sketch of method steps of a method of laserprinting.

FIG. 10 shows a principal sketch of a third arrangement of laser modulesof the laser printing system.

FIG. 11 shows a principal sketch of a 3D printing system for additivemanufacturing.

FIG. 12 shows a principal sketch of a top view of the 3D printing systemfor additive manufacturing.

FIG. 13 shows a principal sketch of the first arrangement of lasermodules and the respectively associated printing areas in the workingplane.

FIG. 14 shows a principal sketch of an embodiment of an optical elementassociated with a laser module.

FIG. 15 shows a principal sketch of an alternative arrangement of laserlight sources in an array of laser light sources.

FIG. 16a shows a principal sketch of an arrangement of laser lightsources in an array and an associated integrated intensity profile ofthe array.

FIG. 16b shows an arrangement of laser arrays according FIG. 16a in alaser module as depicted in FIG. 4 with a pattern of on/off switchedpixels and an associated integrated intensity profile.

FIG. 17a shows a principal sketch of an arrangement of the laser lightsources in an array similar to FIG. 15 and an associated integratedintensity profile of the array.

FIG. 17b shows an arrangement of laser arrays according to FIG. 17a in alaser module as depicted in FIG. 4 with a pattern of on/off switchedpixels and an associated integrated intensity profile.

In the Figures, like numbers refer to like objects throughout. Objectsin the Figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the invention will now be described by means ofthe Figures.

FIG. 1 shows a principal sketch of a first laser printing system 100.The laser printing system 100 comprises two laser arrays 110 withsemiconductor lasers 115 and an optical element 170. The semiconductorlasers 115 are VCSELs which are provided on a semiconductor chip. Inthis case all VCSELs 115 of one array 110 are provided on one chip. Theoptical element 170 is an imaging lens with a focal distance f. Thearrays 110 have a width D perpendicular to the plane of the drawingswhich is diffusely imaged to a working plane 180 by means of the imaginglens. The width d of the diffuse image of each array 110 with the widthD in the working plane 180 defines the width of a pixel in the workingplane 180. The width of the pixels d is smaller than the width D of therespective array. The images of the arrays are thus demagnified. Thedistance b between the working plane 180 and the imaging lens or opticalelement 170 is bigger than the focal length f of the imaging lens. Theoptical element 170 or imaging lens defines together with the workingplane 180 an object plane 150 in distance g bigger than the focal lengthof the imaging lens. The light emitting surfaces of the VCSELs 115 arenot arranged in the object plane but behind the object plane in adistance such that no sharp projection of the light emitting surfaces ofthe VCSEL 115 is provided. The distance a between the light emittinglayers of the VCSELs 115 and the object plane is chosen in a way thatthe laser light of at least two VCSEL 115 of one laser array 110simultaneously illuminate an area element of a pixel. FIG. 2 shows thearrangement of a divergence angle of laser light emitted by one VCSEL115 in relation to the object plane 150 in more detail. The divergenceangle of the VCSELs 115 is given by an angle α as shown in FIG. 2 anddefines the cone of laser light emitted by the single VCSEL 115. TheVCSELs 115 in the laser array 110 do have a distance p with respect toeach other (pitch). The relation between pitch p and distance a has tofulfill the condition:a≥p*(tan α)⁻¹

Laser light emitted by the VCSELs 115 of the laser array 110 overlap inthe object plane 150 such that each area of the same size as the laserarray 110 in the object plane 150 is illuminated by means of at leasttwo VCSEL 115. Each area element of the pixel defined by the pixel sized is consequently also illuminated via the imaging lens by means of atleast two VCSELs 115 of the respective laser array 110. The VCSELs ofeach laser array are driven in parallel and thus emit laser light at thesame time. The size of the pixel is given byd=M*D,wherein the magnification M is given byM=b/g.

The image of the laser array 110 in the working plane 180 is diffuse inorder to increase the homogeneity of the energy input to the object inthe working plane 180 and improve the reliability with respect tomalfunctions of single VCSEL.

The total distance between the laser arrays 110 of the laser module andthe working plane 180 may be increased by means of a micro-lens array175 which may be combined with the laser array 110 as shown in FIG. 3.The micro-lens array 175 may be arranged between the laser array 110 andthe object plane 150 in order to decrease the divergence angle α of eachVCSEL 115. The distance a and therefore the total distance to theworking plane 150 has to be increased in order to fulfill the conditiona≥p*(tan α)⁻¹ if the pitch of the VCSELs 115 remains the same.

In an improvement of the condition discussed with respect to FIG. 2 maybe achieved by taking into account an active diameter v of the VCSELs115 in case of VCSELs 115 with circular aperture. The active diameter vcorresponds to the diameter of the light emitting area of the activelayer. The relation between active diameter v, pitch p and distance ahas in this improved embodiment to fulfill the condition:a≥(p−v)*(2 tan α)⁻¹.

FIG. 4 shows a principal sketch of an arrangement of laser arrays 110 ina laser module of the laser printing system 100. The laser or VCSELarrays 110 are not quadratic but rectangular, with the long side of therectangle being arranged in the direction of the movement of the object(see FIG. 5). This allows higher total powers per pixel, withoutreducing the resolution in the lateral direction. The VCSEL arrays 110are further arranged in two columns which are slightly shifted withrespect to each other (cascaded or staggered arrangement). This enablesa defined overlap with respect to the illumination of area elements ofthe object if the object moves perpendicular to the direction of thecolumns of VCSELs.

FIG. 5 shows a principal sketch of a first arrangement of laser modulesof the laser printing system 100. The laser modules comprise staggeredor cascaded arrangements of laser arrays 110 as shown in FIG. 4 and anoptical element 170. The optical element 170 images all laser arrays 110of the respective laser modules to the working plane 180 of the laserprinting system 100. The optical element 170 defines the total size Y ofthe laser module wherein the width of the arrangement of laser arrays110 of the respective laser module defines the printing width y of onelaser module. The laser modules are arranged in columns parallel to eachother wherein each column is shifted such that a continuous area can beilluminated in the working plane 180 if the object moves in direction250 relative to the laser modules. The printing area can thus be adaptedto the size of the object in the working plane independent on the size Yand printing width y of the single laser module. The number of columnsneeded in order to continuously illuminate an object moving in theworking plane 180 depends on the size Y and printing width y of thelaser modules. The laser modules within one column are separated by atleast by a distance Y such that at least N=Y/y columns are needed. Thecascaded optical elements 170 may be fabricated as a single piece e.g.by glass molding. Alternatively, a lens array may be assembled fromindividual lenses by active or passive alignment.

FIG. 6 shows a principal sketch of a second arrangement of laser modulesof the laser printing system. The arrangement is quite similar to thearrangement discussed with respect to FIG. 5. The laser arrays 110 ofthe laser modules are slanted (rotated around their center) with respectto a direction perpendicular to the direction of movement 250 of theobject relative to the laser modules. This enables integrated intensityprofiles with smooth slopes as shown in FIGS. 7 and 8, which may alsooverlap with the adjacent pixels, to improve the homogeneity of thetotal intensity distribution, especially if the pixels are slightlymisaligned with respect to each other.

FIG. 7 shows an integrated intensity profile in a direction 610perpendicular to the direction of movement 250 of the object relative tothe laser modules with every second pixel off in the arrangement oflaser modules shown in FIG. 6. The pixel profile is almost triangular,with large slopes that overlap with the adjacent pixels. FIG. 8 shows anintegrated intensity profile with an arbitrary pattern of on/offswitched pixel in the arrangement of laser modules shown in FIG. 6. Thenumbers “1” and “0” indicate which of the adjacent laser arrays 110 areswitched on or off. The integrated intensity profile shows the overlapof two or more neighboring pixels in the working plane 180.

FIG. 9 shows a principal sketch of method steps of a method of laserprinting. The shown sequence of steps does not necessarily imply thesame sequence during execution of the method. Method steps may beexecuted in different order or in parallel. In step 910 the object likea sheet of paper is moved in the working plane of the laser printingsystem relative to the laser module. In step 920 laser light is emittedby means of the laser module comprising at least two laser arrays ofsemiconductor lasers and at least one optical element. In step 930 thelaser light emitted by the laser arrays is imaged, such that laser lightof semiconductor lasers of one laser array is imaged to one pixel in theworking plane and an area element of the pixel is illuminated by meansof at least two semiconductor lasers. The object may be moved and at thesame time laser light of the laser arrays may be emitted and imaged tothe working plane.

When using individually addressable lasers or laser arrays, the maximumspeed in the 3D printing process can be obtained when along a line allindividual pixels can be written at the same time, i.e. by a separatelaser or laser array per pixel. Typical line widths in a laser printingsystem or machine are in the order of 30 cm or more. On the other hand,the size or printing width of a laser module of individually addressablelasers or laser arrays is limited to a few cm. These laser modulescorrespond usually to one micro-channel cooler on which the lasermodules are arranged.

It is therefore necessary to use a number of laser modules andcorresponding micro-channel coolers and to stack them together to acomplete laser printing module or printing head. Alignment tolerancesbetween neighboring micro-channel coolers with laser modules may resultin a gap in the working plane 180 to which no or not sufficient laserlight can be provided. In worst case such a gap leads to defects withrespect to processing of the object as printed sheets of inferiorquality or in the parts produced by means of a 3D printer/additivemanufacturing machine.

In view of the typical size of a laser light source 116 of 100 μm andthe fact that several alignment tolerances add up together, the problemof a gap is a severe issue. Even with tight tolerances in eachindividual step of assembling the laser printing system, the overalltolerance chain can lead to significant deviations of 30 μm or more.

It may be advantageous in this respect not only to provide overlappingintensity distributions but to use additional laser light sources 116 atthe edge of each laser module. Said laser light sources 116 are socalled overlap laser light sources 117 which are arranged such that thelight of these overlap laser light sources 117 overlaps with light oflaser light sources 116 of neighboring laser module. This means that thepitch between neighboring laser modules is smaller than the totalprinting width of the laser module by at least the width of one laserlight source 116 (e.g. 100 μm).

If the maximum tolerance from the mechanical/optical alignment ofneighboring laser modules is smaller than the width of one laser lightsource 116, it is sufficient to have—by design—an overlap of one laserlight source 116 in order to avoid gaps in the working plane to which nolaser light can be provided. Anyhow, it may alternatively be possible toprovide more than one overlap laser light sources 117 if the maximumtolerance from the mechanical/optical alignment of neighboring lasermodules is bigger than the width of one laser light source 116. It mayin this case be possible to use the overlap laser light sources 117 inaccordance with the width of the gap between neighboring laser modules.The laser printing system may in this case be calibrated such that theoverlap laser light sources 117 fill the unintended gap between thelaser modules. Depending on the gaps and the width of one laser lightsource 116 it may be that one, two, three or even more of the overlaplaser light sources 117 are used in order to enable a continuous, i.e.seamless illumination of the working plane.

FIG. 10 shows an embodiment of such an arrangement with overlap laserlight sources 117 which are arranged in an overlapping arrangement ofneighboring laser modules which are laser sub-modules 120 in order tocompensate potential misalignment of laser submodules 120 with respectto each other. The overlap laser light sources 117 are indicated by aline pattern.

The printing width of neighboring laser sub-modules 120 overlap by acomplete laser light source 116 or more explicit overlap laser lightsource 117. A laser light source 116 may comprise different as theprevious embodiments only a single laser or in accordance with theprevious embodiments a laser array such as laser arrays 110. The singlelasers may comprise optical elements like micro-lenses. In case of laserarrays micro-lens arrays may be comprised. The arrangement of the lasersub-modules 120 is similar to the arrangement as shown in FIG. 5. Thelaser modules shown in FIG. 5 are arranged such that each laser array110 illuminates a dedicated pixel or area element in the working plane180. The laser sub-modules 120 as shown in FIG. 10 are arranged suchthat in case of no alignment errors during assembly the overlap laserlight sources 117 are adapted such that they can illuminate the samearea element in the working plane 180 as a laser light source 116 of aneighboring laser sub-module 120.

FIGS. 11 and 12 show schematically an embodiment of a 3D-laser printingsystem for additive manufacturing. Referring to FIG. 11, the 3D-laserprinting system includes a process chamber 300 with a support 400 forcarrying building material and a three-dimensional article 500 to bebuilt thereon. On the support 400 a building platform 450 may beprovided which serves as a removable base for removing thethree-dimensional article 500 after the building process is finished. Itshall be noted that the building platform 450 may also be omitted. Aboundary structure 470, such as vertical walls, may be arranged aroundthe support 400 to confine layers of the building material on thesupport 400. The boundary structure may be arranged as a removableframe, which may include a vertically movable base which is removablyattached to the support 400, similarly to the building platform 450. Asillustrated in FIG. 12, a building area 480 may be defined by theboundary structure 470. The building area 480 may have a rectangularcontour as shown in FIG. 12 or any other contour such as but not limitedto a square-shaped or a circular contour.

Above the support 400, an illumination unit 700 is arranged. Preferably,the illumination unit 700 is movable across the building area 480 in adirection depicted by the arrow in FIG. 12 which is the direction ofmovement 250 in this embodiment. The illumination unit 700 may beconfigured to be moved back in an opposite direction. It may be switchedon or switched off during the back movement.

The support 400 is movable up and down relative to the illumination unitin a vertical direction, i.e. in a direction perpendicular to thedirection of movement 250 of the illumination unit 700. The support 400is controlled in such a manner that an uppermost layer of the buildingmaterial forms the working area 180.

The 3D-laser printing system further includes a control system 800 forcontrolling various functions of the 3D-printing system. A recoatingdevice (not shown) may be provided to apply layers of building materialonto the building platform 450 or the support 400 or the movable base ofa removable frame (not shown). Furthermore, one or more separate heatingdevice(s) (not shown) may be provided that may be used to heat anapplied layer of building material to a process temperature and/or tocontrol the temperature of the building material in the boundarystructure 470, if necessary.

The building material preferably is a powder material that is configuredto transform under the influence of the laser light emitted by the laserlight sources into a coherent mass. The transformation may include, forexample, melting or sintering and subsequent solidification and/orpolymerization in the melt. Preferably, the building material is aplastic powder, for example a thermoplastic powder. Examples of suchplastic powders are PA 12 (polyamide 12) or other polyamides,polyaryletheretherketone, such as PEEK or other polyetherketones. Thepowder may also be a powder from a metal or a metal alloy with orwithout a plastic or metal binder, or a ceramic or composite or otherkind of powder. Generally, all powder materials that have the ability totransform into a coherent mass under the influence of the laser lightemitted by the semiconductor lasers can be used. The building materialmay also be a paste-like material including a powder and an amount ofliquid. Typical medium grain sizes of the powder lie between 10 μm oreven less and 100 μm, measured using laser diffraction according to ISO13320-1.

Typical wave length of the laser light sources are preferably 980 or 808nm in conjunction with absorbers (laser light absorbing additives to thepowder material), e.g. but not limited to Carbon Black, suitable toenable a sufficient absorption of the chosen wave length. In principleany wavelength is possible as long as a suitable absorber material canbe added to the powder material. Typical layer thicknesses of the powderlayers may range between about 10 μm and about 300 μm, in particular forplastic powders, and about 1 μm up to about 100 μm, in particular formetal powders.

The illumination unit 700 will be described more in detail withreference to FIG. 11 to 13. FIG. 13 shows an arrangement of lasermodules similar to that of FIG. 5 with the difference that more than twocolumns and the demagnified image produced by the laser modules with theoptical elements in the working plane 180 are shown. FIG. 13 shall notbe considered as a perspective view but only as a schematic sketchdepicting the arrangement of modules and the corresponding demagnifiedimages. As schematically depicted in FIG. 13, the illumination unit 700includes a plurality of laser modules 200 arranged in columnsperpendicular to the direction of movement 250. Like in FIGS. 5 and 6,the columns of the laser modules are staggered with respect to eachother such that a first laser module 200 ₁ of a first column c1 of lasermodules is adapted to illuminate a first area y1 of the powder in theworking plane 180. The second module 200 ₂ of a second column c2 oflaser modules is adapted to illuminate a second area y2 of the powder inthe working plane 180, wherein the first area y1 is adjacent to thesecond area y2 such that continuous, i.e. seamless illumination of theobject is enabled, By means of this, the illuminated areas y1, y2 in theworking plane 180 form a contiguous area in the direction perpendicularto the direction of movement. As further depicted in FIG. 13, lasermodules that are staggered in the direction of movement 250 formcascades. A first cascade k1 is formed by the first laser modules 200 ₁,200 ₂, 200 _(n) of the columns A second cascade k2 ist formed by thesecond laser modules 201 ₁, 201 ₂, 201 _(n) of the columns and so on.The number of cascades is such that the sum of the individual printingwidths y in a direction perpendicular to the direction of movement 250covers the width of the building area 480. For different 3D-laserprinting systems having different building areas, the number of cascadescan be easily adapted to cover the different widths of the respectivebuilding areas 480. In a typical example of a 3D-laser printing systemfor additive manufacturing using VCSEL as semiconductor lasers, onearray may have several hundreds of semiconductor lasers, for exampleVCSELs, one module may include 2×16=32 arrays, one cascade may include 9modules and the illumination unit may include several of those cascades,for example 3. This typically allows to illuminate a building area 480of about 84 mm. Other building areas can be achieved by selectingappropriate numbers of modules per cascade and of cascades. As describedabove, one single optical element 170 is associated with one module andone module is preferably used to illuminate 16, 32 or 64 pixel in theworking plane.

Referring again to FIG. 11, since the illumination unit 700 is arrangedwithin the process chamber, it is exposed to the ambient conditions thatexist in the process chamber 300, such as the average temperature,temperature gradients, vapors, gas flows, such as inert gas flows, dust,splashes of molten material which could emerge from the building area,monomers emerging from the transformation process of the buildingmaterial and moving around in the process chamber etc. A distancebetween the outermost optical element of the illumination unit 700 thatis facing towards the building area and the working plane 180 may be inthe range between about 5 mm to about 50 mm. This arrangement of theillumination unit 700 is different from the known laser-melting orlaser-sintering machines. To protect the illumination unit 700, aprotective device 750 is arranged on a side of the illumination device700 facing the support 400. The protective device 750 may be realized byat least one plate that is transparent for the laser light. Thetransparent plate may be integrally formed with the illumination device700. In particular, the protective device 750 may be a glass plate.Moreover, the protective device 750 may be a single piece protecting allmodules of the illumination unit 700 or may be composed of a pluralityof pieces, one for each module. A distance between the outermost surfaceof the protective device and the working area may be only severalmillimeters, for example, about 5 mm. More generally, if a specificdemagnification of n:1 is intended, a distance between the laseremitting portion of the semiconductor lasers and the outermost opticalelement (in the optical path) may be essentially about n times thedistance between the outermost optical element and the working area 180.

Preferably, a temperature control device (not shown) is associated withthe protective device 750. The temperature control device may berealized in the form of a number of (i.e. one or more) heating elements.Preferably, the heating elements are arranged on the transparent plate,in particular only on such positions, where effectively no laser lightis transmitted or where no laser light is intended to be transmitted.More preferably, the heating elements are provided on a side of theprotective device 750 that faces away from the support 400, i.e. thatfaces towards the laser light sources of the illumination unit 700. Thisfacilitates cleaning of the protective device and reduces abrasive wearof the heating elements. The heating elements may be in the form of heatconductive paths. In particular, the heating elements may bevapor-deposited or provided in the transparent plate duringmanufacturing of the protective device. In a further modification, theprotective device 750 may include an assembly of two or more glassplates with vacuum or gas in-between the plates for thermal isolation.With such a design, a heat flow into the interior of the illuminationunit 700 can be reduced or even prevented. In the case of an assembly ofplates, the heating device may be provided at an inner side of one platefacing towards an adjacent plate, in particular of the outermost platefacing towards its adjacent plate.

The temperature control device controls the temperature of theprotective device 750 in such a manner that the temperature is adjustedto a specific temperature preferably in a range between around a few(preferably 10 at most, more preferred 5 at most and most preferred 3 atmost) Kelvin below the process temperature to a few (preferably 10 atmost, more preferred 5 at most and most preferred 3 at most) Kelvinabove the process temperature. Due to the energy consumption and limitedefficiency of the semiconductor lasers, the illumination unit 700 iscooled and preferably held at a temperature that can be considerablylower than the process temperature of the transformation process of thebuilding material, depending on the building material used. Hence, heatloss by thermal radiation from the layers of building material to theillumination unit 700 is reduced or prevented. Moreover, the forming ofcondensates at the surface of the protection device 750 can be reducedor avoided. Those condensates would reduce the transparency of the glassplate/laser window/protective device and therefore would reduce thedisturbance and/or the amount of absorbed laser light energy at thesurface of the powder material. As a consequence, the quality of thethree-dimensional articles to be built would be decreased. Thetemperature control device therefore ensures good quality of thethree-dimensional articles to be built.

The presence of the protective device 750 requires the image distance b,i.e. the distance between the optical element 170 and the working plane180 (see FIG. 1), to be a certain minimum image distance. Due to thenecessary demagnification, the object distance g, i.e. the distancebetween the object plane 150 and the optical element 170, is relativelyhigh. The divergence angle α of each VCSEL 115 results in the fact thatthe beam path of VCSEL-arrays of adjacent modules cross each other whichrenders a module-wise imaging onto the object plane 150 difficult. Toavoid this, the illumination unit 700 includes micro lens arrays 175 asdepicted in FIG. 3 for each module.

Preferably, the laser arrays 110 of the modules 200 are arranged asdepicted in FIG. 14. In a further preferred embodiment, an opticalelement 170 associated with such an arrangement of the laser arrays 110has a contour obtained from a circular or rotationally symmetricalcontour, which is truncated on opposing sides and wherein the opposingsides 1 of the optical element 170 are aligned with respect to eachother along an axis which is preferably orientated in a directionperpendicular to the direction of movement 250. More precisely, in thecase of the arrangement of the laser arrays as in FIG. 14, the opticalelement 170 has the contour of a modified rectangle with two opposingcircular segment-shaped short sides s that connect the parallel longsides 1. This takes into account that a circular optical element wouldnot be fully illuminated with the rectangular arrangement of the laserarrays as depicted in FIG. 14. Hence, the portions of a circular opticalelement that are not fully illuminated can be omitted. By means of theshape of the optical element 170, the size of a module in the directionof movement 250 can be reduced. As a result thereof, the size of theentire illumination unit 700 in the direction of movement 250 can bereduced. This has the advantage that a line oriented in the direction ofmovement can be illuminated within a reduced time which enhances theproductivity of the whole 3D-printing system. Also, neighboring pixelsat the border between one module 200 ₁ and a neighboring module 200 ₂ ofone cascade k1 and/or of one module 200 _(n) of one cascade k1 and aneighboring module 201 ₁ of a neighboring cascade k2 can be illuminatedwith reduced time offset. This also increases the quality of thethree-dimensional article.

The arrangement of the VCSELs in the laser array 110 defines theintensity profile. If the arrangement is substantially rectangular, i.e.the VCSELs are arranged in the array in rows and columns, the integratedintensity profile 600 of the array is substantially rectangular, i.e.the integrated intensity profile has a so-called “flat top” profile asdepicted in FIG. 16a . In a module according to FIG. 4, where severalarrays 110 are switched on and several arrays are switched off, theintegrated intensity of the module in a direction 610 perpendicular tothe direction of movement 250 is as shown in FIG. 16b , i.e. has sharpedges (in the case that the object plane 150 is coincident with theactive area of the semiconductor lasers).

It may be desirable to have an integrated intensity profile withoutsharp edges. This can be achieved by an arrangement according to FIG.15, wherein the VCSELs in one array 110 are positioned in rows andcolumns and wherein the outer contour of the array is substantiallypolygonal, in particular, substantially hexagonal. The individual VCSELsare positioned at grid points that are staggered from one column to thenext column, wherein the columns are oriented perpendicular to thedirection of movement 250. Preferably, the outer contour of the arrayhas a hexagonal shape with two opposing parallel sides p which extendperpendicular to the direction of movement 250.

As depicted in FIG. 17a , the integrated intensity profile 600 of alaser array with a substantially hexagonal shape as shown in FIG. 15,has rounded edges and is similar to a Gaussian intensity distribution.For a laser module with switched on/off arrays, the integrated intensityprofile 600 along a direction 610 comprises rounded transitions asdepicted in FIG. 17b . Hence, deviations from an average value ofintensity are smaller.

With the illumination unit 700, one pixel in the working area isilluminated by a multitude of semiconductor lasers of a laser array 110at the same time. The total number of semiconductor lasers may beselected such that failure of less than a predetermined number of thesemiconductor lasers reduces the output power of the laser array 110only within a predetermined tolerance value. As a result thereof, therequirements with respect to the working life of the individual VCSELsmay not be unusually high.

The individual VCSELs of a laser array may be grouped in sub-groups withrespect to their addressability by control signals. A sub-group mayinclude at least two VCSELs. At least two sub-groups of VCSEL of onelaser array may be individually addressable such that an output power,i.e. an intensity, of the laser array 110 is controllable by switchingoff one or more sub-groups of VCSEL. Also, an embodiment may be providedwhere the semiconductor lasers of one laser array are individuallyaddressable so that an output power of the laser array may be controlledby switching on/off individual semiconductor lasers.

In a further embodiment, the semiconductor lasers or the laser arrays ofthe illumination unit 700 can be further controlled such that asemiconductor laser or a laser array which is not used for illuminatingcan optionally be used for providing heat to the building material inthe working plane 180. To accomplish this, a control device is providedwhich controls the semiconductor lasers individually or the laser arraysin such a manner that the semiconductor lasers or a laser array which isnot used for illuminating emits less intensity as required fortransforming the building material so as to only heat the buildingmaterial in the working plane. This heating can be used in addition tothe separate heating device described above or as an exclusive heatingsystem that pre-heats the building material to a working temperature.

The illumination unit 700 may include overlap light sources 117 asexplained with reference to FIG. 10. The overlap light sources 117 arepreferably provided at the border between one module of one column to aneighboring module of a neighboring column, for example module 200 ₁ ofcolumn c1 and module 200 ₂ of column c2 in FIG. 13 and/or from onemodule in one cascade to a neighboring module in a neighboring cascade,for example module 200 _(n) in cascade k1 and module 201 ₁ in cascade k2in FIG. 13. The overlap light source 117 balances the energy lossresulting from a time offset of adjacent pixels perpendicular to thedirection of movement 250 due to the staggered arrangement of a moduleand/or due to the cascaded arrangement of the modules.

The overlap light sources 117 can be controlled in such a manner thatenergy losses due to time offset and/or energy losses or energy excessesdue to misalignment of VCSELs or arrays can be compensated. Hence, thesum of energy that is provided to the working area by overlap lightsources 117 can be adjusted to be the energy necessary for illuminatingin the case of time offset zero and/or perfectly aligned VCSELs orarrays. The energy provided by the overlapping VCSELs or arrays can beselected depending on the type of building material. Influencing factorsmay be the heat conductivity of the powder bed, the heat conductivity ofthe melt or the sintered mass, the particle size, etc.

In a further modification, the semiconductor lasers of the illuminationunit are realized by VECSELs (Vertical External Cavity Surface EmittingLaser).

The 3D-printing system described above is operated as follows. Layers ofthe building material are successively deposited onto the support 400 orthe building platform 450 or a previously illuminated layer such thatthe new layer of building material forms the working plane 180. Then,the illumination unit 700 moves across the building area 480 in thedirection of movement 250 and selectively illuminates the buildingmaterial in the working area 180 at positions corresponding to thecross-section of the three-dimensional article in the respective layer.After one layer has been illuminated, the support is moved downward suchthat the new layer can form the working area 180.

While the invention has been illustrated and described in detail in thedrawings and the foregoing description, such illustration anddescription are to be considered illustrative or exemplary and notrestrictive.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art and which may be usedinstead of or in addition to features already described herein.

Variations to the disclosed embodiments can be understood and effectedby those skilled in the art, from a study of the drawings, thedisclosure and the appended claims. In the claims, the word “comprising”does not exclude other elements or steps, and the indefinite article “a”or “an” does not exclude a plurality of elements or steps. The mere factthat certain measures are recited in mutually different dependent claimsdoes not indicate that a combination of these measures cannot be used toadvantage.

Any reference signs in the claims should not be construed as limitingthe scope thereof.

LIST OF REFERENCE NUMERALS

-   100 laser printing system-   110 laser array-   115 semiconductor laser-   116 laser light source-   117 overlap laser light source-   120 laser sub-module-   150 object plane-   170 optical element-   175 micro-lens array-   180 working plane-   200, 200 ₁, 200 ₂, 200 _(n)-   201 ₁, 201 ₂, 201 _(n) laser modules 250 direction of movement-   300 process chamber-   400 support-   450 building platform-   470 boundary structure-   480 working area-   500 three-dimensional article-   600 integrated intensity-   610 direction perpendicular to direction of movement-   700 illumination unit-   750 protective device-   800 control unit-   910 method step of the object-   920 method step of emitting laser light-   930 method step of imaging the laser light

The invention claimed is:
 1. A laser printing system for building anobject in layerwise fashion through additive manufacturing, wherein thelaser printing system comprises: a plurality of laser modules, each ofthe laser modules comprising at least two laser arrays with each laserarray having a plurality of semiconductor lasers, and at least oneoptical element arranged to focus laser output from at least one of thelaser arrays to a working plane within which the object is built, thesemiconductor lasers in each of the laser arrays being set in a rowacross a face of at least one of the laser modules whereby there are atleast two rows presenting laser output across the laser module facearranged in a two-dimensional pattern of parallel rows, the opticalelement imaging laser light emitted by the laser arrays, such that thelaser light from the semiconductor lasers of one of the laser arrays isimaged to one pixel in the working plane, and wherein the laser printingsystem further comprises a process chamber comprising a support forlayers of a material, wherein the laser modules are arranged in theprocess chamber and wherein a protective device is arranged at a side ofthe laser modules which faces the support.
 2. The laser printing systemaccording to claim 1, wherein an area element of the pixel isilluminated by means of at least two of the semiconductor lasers.
 3. Thelaser printing system according to claim 1, wherein the optical elementis arranged in a way that an object plane of the optical element withrespect to the working plane does not coincide with a plane of thesemiconductor lasers such that cones of laser light emitted by adjacentsemiconductor lasers overlap in the object plane.
 4. The laser printingsystem according to claim 1, wherein the laser module comprises three ormore laser arrays.
 5. The laser printing system according to claim 1,wherein the optical element comprises one lens being adapted to imagethe laser light of the laser arrays to the working plane.
 6. The laserprinting system according to claim 1, wherein the optical element isadapted such that images from the laser arrays overlap in the workingplane.
 7. The laser printing system according to claim 1, wherein thelaser arrays of the laser module are arranged in columns perpendicularto a direction of movement of the object in the working plane, thecolumns are staggered with respect to each other such that a first laserarray of a first column of laser arrays is adapted to illuminate a firstarea of the object and a second laser array of a second column of laserarrays is adapted to illuminate a second area of the object, wherein thefirst area is adjacent to the second area such that continuousillumination of the object is enabled.
 8. The laser printing systemaccording to claim 1, wherein the laser arrays are rectangular with thelong side of the rectangle being arranged parallel to a direction ofmovement of the object in the working plane.
 9. The laser printingsystem according to claim 1, wherein the number of columns of lasermodules is arranged in a way such that the distance between lasermodules in one column of laser modules is minimized.
 10. The laserprinting system according to claim 1, wherein the laser arrays of eachlaser module are arranged in an elongated arrangement with the long sideof the elongated arrangement being arranged perpendicular to thedirection of movement of the object in the object plane.
 11. The laserprinting system according to claim 1, wherein the laser arrays of eachlaser module are arranged in an elongated arrangement with the long sideof the elongated arrangement being arranged slanted to the directionbeing perpendicular to the direction of movement of the object in theworking plane.
 12. The laser printing system according to claim 1,wherein the optical element is arranged to demagnify the image from thelaser arrays in the working plane.
 13. The laser printing systemaccording to claim 1, wherein each laser array comprises a micro-lensarray, the micro-lens array being arranged to lower the divergence ofthe laser light emitted by the semiconductor lasers.
 14. The laserprinting system according to claim 1, the plurality of laser modulescomprising at least a first and a second laser module arranged next toeach other, each of the at least first and second laser modulescomprising the at least two laser arrays, wherein at least one of thetwo laser arrays of the first or the second laser module is arranged asan overlap laser light source such that in operation at least onedefined area element in the working plane can be illuminated by theoverlap laser light source and a separate laser array of the lasermodule arranged next to the laser module comprising the overlap laserlight source.
 15. The laser printing system according to claim 14,wherein the total energy which is provided to the at least one definedarea element of the object is such that essentially the same energy isprovided per area element as in the case of aligned laser moduleswithout an overlap laser light source.
 16. The laser printing systemaccording to claim 1, wherein the one pixel is illuminated by amultitude of the semiconductor lasers of the laser array at the sametime and wherein the total number of semiconductor lasers is such thatfailure of less than a predetermined number of the semiconductor lasersreduces the output power of the laser array only within a predeterminedtolerance value.
 17. The laser printing system according to claim 1,wherein at least one of the laser modules is configured to illuminate atleast 2 or more pixels using a single optical element associated withthe laser module.
 18. The laser printing system according to claim 7,wherein the optical element associated with one of the laser modules hasan outer contour obtained from a circular or rotationally symmetricalcontour which is truncated on two opposing sides and wherein theopposing sides are aligned with respect to each other along an axiswhich is oriented in a direction perpendicular to the direction ofmovement.
 19. The laser printing system according to claim 1, wherein acontrol device is provided which controls the semiconductor lasersindividually or the laser array in such a manner that at least one ofthe semiconductor lasers or at least one of the laser arrays which isnot used for illuminating is used for providing heat to the workingplane.
 20. The laser printing system according to claim 19, wherein thesemiconductor laser or the laser array which is not used forilluminating is operated with a lower power than another one of thesemiconductor lasers or another one of the laser arrays which is usedfor illuminating.
 21. The laser printing system according to claim 1,wherein at least two of the semiconductor lasers of one of the laserarrays or at least two sub-groups of semiconductor lasers of one of thelaser arrays are individually addressable such that an output power ofthe laser array is controllable by switching off one or moresemiconductor lasers or the sub-groups of semiconductor lasers.
 22. Thelaser printing system according to claim 1, wherein the semiconductorlasers of one of the laser arrays are arranged such that the outercontour of the array has a polygonal or a hexagonal shape.
 23. The laserprinting system according to claim 1, wherein the protective device isformed of at least one plate that is transparent to the laser light. 24.The laser printing system according to claim 1, wherein a temperaturecontrol device is provided that controls the temperature of at least thesurface of the protective device oriented towards the support.
 25. Thelaser printing system according to claim 24, wherein the temperaturecontrol device is configured to heat the protective device such thatthermal radiation from the material in the working plane to theprotective device is substantially prevented.
 26. The laser printingsystem according to claim 1, wherein the laser modules form anillumination unit configured to solidify the material at locationscorresponding to the cross section of the object to be formed in eachlayer.
 27. The laser printing system according to claim 26, wherein thematerial is a powder.
 28. The laser printing system according to claim1, wherein the laser modules form an illumination unit and wherein theillumination unit is configured to move across the working plane. 29.The laser printing system according to claim 1, wherein thesemiconductor lasers are VCSEL (Vertical Cavity Surface EmittingLasers).
 30. The laser printing system according to claim 1, wherein thesemiconductor lasers are VECSEL (Vertical External Cavity SurfaceEmitting Laser).
 31. Method of laser printing, the method comprising thesteps of moving an object in a working plane relative to a laser module;emitting laser light by means of the laser module comprising at leasttwo laser arrays of semiconductor lasers and at least one opticalelement; and imaging the laser light emitted by the laser arrays bymeans of the optical element, such that the laser light of thesemiconductor lasers of one of the laser arrays is imaged to one pixelin the working plane; wherein the laser printing is performed via a 3Dprinting system for additive manufacturing and wherein two or more lasermodules are used which are arranged in columns perpendicular to adirection of movement of the object in the working plane, and whereinthe columns are staggered with respect to each other such that a firstlaser module of a first column of laser modules is adapted to illuminatea first area of the object and a second laser module of a second columnof laser modules is adapted to illuminate a second area of the object,wherein the first area is adjacent to the second area such thatcontinuous illumination of the object is enabled.
 32. The laser printingsystem according to claim 1, wherein the laser modules are arranged incolumns mounted to and carried by a base which is movable in a directionof movement across the working plane; and wherein the module columns arestaggered with respect to each other such that a first laser module of afirst column of the laser modules provides laser output to illuminate afirst area of the working plane and a second laser module of a secondcolumn of the laser modules provides laser output to illuminate a secondarea of the working plane, wherein the first area is at least contiguousto the second area enabling coverage of the working plane encompassed bythe first and second areas.
 33. The laser printing system according toclaim 1, wherein the laser arrays are controllable to be switched on andoff such that the pixels in the working plane located at locations thatcorrespond to a respective cross section of an object thereby generate alayer constituting that cross section by such controlled switching. 34.The laser printing system according to claim 14, wherein a total energywhich is provided to at least one defined area element in the workingplane is such that essentially the same energy is provided per areaelement as in the case without a time offset between the illumination ofthe at least one defined area element by the laser array and thecorresponding overlap laser light source.
 35. The laser printing systemaccording to claim 14, wherein an adapted intensity of the laser arrayand/or the corresponding overlap laser light source is such that anenergy loss in the time between the illumination by the laser array andthe illumination by the corresponding overlap light source of a definedarea element in the working plane that is illuminated by the laser arrayat a time t1 and by the overlap laser light source at a time t2 or viceversa is compensated.
 36. The laser printing system according to claim14, wherein the overlap light source balances the energy loss resultingfrom a time offset of adjacent pixels perpendicular to the direction ofmovement due to the staggered arrangement of a module and/or due to acascaded arrangement of the modules.
 37. The laser printing systemaccording to claim 1, wherein the protective device is formed of a glassplate.